1. Field of the Invention
The present invention relates to a socket connector, and more particularly to a socket connector incorporated with a contact terminal having a layer of solder affinity and oxidation retarding preparation adjacent to a tail portion of the contact terminal as well as a solder ball attached thereto so as to achieve perfect solder joint.
2. Description of the Related Art
Soldering between a solder tail of a contact terminal and a conductive pad on a printed circuit board is comparably reliable and commonly practiced in the electrical connector field. When conducting a soldering process, there is a dilemma. On one hand, it is requested that the solder tail expresses solderable property, i.e. the solder can be readily and easily attached thereto. If the solder joint is not properly formed between the solder tail and the printed circuit board, defective interconnection or so called cold-joint will be encounter. Rework process will always be needed to correct this problem. On the other hand, because of this solderable property, the solder tends to flow upward or wick along an external surface of the solder tail. Once the solder flows and wicks upwardly along the surface resulted from the capillary force, the overall characteristic of the contact terminal will be changed or negatively modified if the wicking reaches up to a middle portion of a contact terminal. For example, when the contact terminal is designed, intended normal force, deflection, etc. have been carefully calculated so as to meet the field requirements. Once the solder flows and wicks upwardly to cover the contact terminal, the characteristic of the contact terminal will be altered, and the normal force and other properties will be altered accordingly. In worse situation, a connector after soldering will be found failed resulted from this solder wicking. As a result, the contact terminal is requested to provide a mechanism to limit the wicking.
U.S. Pat. No. 4,019,803 (hereinafter US'803) issued to Schnell on Apr. 26, 1977 discloses a solder substrate clip having a contact arm. A mass of solder is secured to the arm on a side away from a contact surface and a solder globule integral with the mass of solder at an edge of the arm extending from the mass across the edge of the arm to the contact surface for engagement with a contact pad on the substrate.
U.S. Pat. No. 4,846,734 issued to Lytle on Jul. 11, 1989 discloses a connector featuring that a connector adapted to be attached to a mother printed circuit board and to removably receive a daughter printed circuit board of the edge card type and adapted to mechanically and electrically couple the mother and daughter printed circuit boards. According to its disclosure, the invention may be incorporated into a method to make the contact terminal and which further includes the step of fabricating the contact of phosphor bronze. The method further includes the step of plating the contact with nickel to a thickness of about between 0.000050 and 0.000150 inches. The method further includes the step of plating the lower portion of the contact with solder of about 60 percent tin and 40 percent lead to a thickness of about between 0.000100 and 0.000500 inches. The method further includes the step of plating the contact portion of the contact with about 40 microinches thick or thicker of PdNi flashed with gold to a thickness of about 0.000004 inches nominally. It is known to the skilled in the art that tin-lead is solderable material, while nickel oxide is non-solderable. Because of that, a tin-lead coating is applied to the lower portion, which according to Lytle, it increases solderablility of the lower portion which is intended to be soldered into a via of a printed circuit board.
U.S. Pat. No. 5,453,017 issued to Belopolsky on Sep. 26, 1995 expressivebly take the advantage of the benefit disclosed in Lytle. In Belopolsky, it discloses an improved connector for an electronic module or the like and includes a housing having a socket opening that is sized and configured to accept an electronic module, and a plurality of terminals mounted to the housing. Each of the terminals has a foot portion having a layer of non-solderable material coated on one side of the foot portion to prevent solder from adhering to that side. An arched capillary nest is formed by a channel surface on the underside of the foot portion when the terminal is mounted on a conductor pad such that solder flows through the capillary nest under the influence of capillary forces from the side of the terminal having a non-solderable coating thereon to the other side for forming a solder joint on that other side. A ring of non-solderable material is coated around a middle portion of the terminal to prevent solder from flowing to the electrical contact surfaces located above the ring. As a result, the connector terminals can be soldered to a printed circuit board or the like in a simple and inexpensive manner and without the formation of known solder defects. As disclosed by Belopolsky, solderable material used in capillary nest is to promote solderability on the solder tail, while the non-solderable ring located at the middle portion limits the solder from wicking further upward. However, it is very difficult to fabricate the capillary nest especially when the dimension of the terminal becomes smaller and smaller.
U.S. Pat. No. 4,722,470 issued to Johary on Feb. 2, 1988 discloses another mechanism to overcome or control the solder wicking. According to Johary, a solder transfer member for applying discrete bodies of solder of predetermined size to the leads of a component for subsequent surface mounting to a substrate. The transfer member is a plate having a non-wetted surface, for example titanium, with an array of cavities matching the component lead pattern, each having a volume corresponding to the desired amount of solder to be applied to the corresponding lead. The method includes placing solder paste on the transfer member and filling the cavities by wiping the plate surface. The component is placed on the transfer member with the leads contacting the solder paste in the cavities. Reflow of the solder paste bonds to each lead a discrete body of solder having a precisely determined size. To limit wicking of solder on the leads, selective masking may be performed by applying a water soluble mask coating to the leads and removing the mask from selected areas by placing the component against a surface charged with water before placing the component on the transfer member.
U.S. Pat. No. 6,042,389 issued to Lemke on Mar. 28, 2000 disclosed another mechanism to limit the solder wicking issue. According to FIG. 6, along with description, “The opening 96 also can function as a thermal break to retard solder wicking, in the same manner as openings 89 in the FIG. 6 embodiment. The terminal 90 may also include passivation or anti-wicking coatings to prevent solder flow toward the contact sections. Aperture or opening 89 defined in the contact tail 76 is used to limit the wicking issue.”
In addition, in the above described Schnell '803 patent, the specification makes another disclosure. According to its description, along with FIGS. 1 to 4, it looks like that Schnell uses energy to control the wicking issue. According to Schnell, the amount of energy supplied to the interface between the solder mass and the arm is sufficient to melt the entire mass, in that way assuring that a relatively large mass of molten solder does not coat the contact surface of the arm. While molten solder does not readily flow across the raw uncoated edges, a relatively large amount of molten solder could flow across the edges and coat the contact surface. This is undesirable because when a substrate is moved into the mouth the arms are bent further apart than intended due to the thickness of the solder coating and may be overstressed. During soldering of the clip to the substrate, the thick layer of solder would be melted freeing the arms for undesired movement during the soldering operation. Overstressed arms may not be strong enough to engage the substrate tightly.
U.S. Pat. No. 4,120,558 issued to Seidler on Oct. 17, 1978 discloses another way, as compared to Lemke and Schnell, to attach the solder mass to the contact. Seidler uses spring fingers to mechanically hold the solder mass, such as shown in FIGS. 1 to 5, and 13 to 15. According to Seidler, each clip includes a flat body portion 15, a pair of spring fingers 16, bent to extend upwardly and laterally from the plane of the body portion distally of the clip and spaced apart by the width of a central spring finger 17 which extends laterally in a position spaced from and substantially parallel to the fingers 16, defining a gap 21 adapted to receive the edge of a substrate (not shown). The fingers 16 and 17 are formed from the blank shown in FIG. 4 by the parallel cuts 18 which terminate at end points 18′. An additional gripping finger 19 is provided by the U-shaped cut 20, the sides of which lie parallel to the cuts 18 and the closed end 20′ being below the line of the ends 18′, this finger thus being formed partially from the material in the central finger 17. The free end of the finger 17 is curved arcuately away from the finger 16, and the gripping finger 19 is curved arcuately toward the curved end of finger 17, in a position to grip securely the short cylindrical slug of solder 22, as clearly shown in FIGS. 1, 2 and 3.
U.S. Pat. No. 6,969,286 issued to Mongold on Nov. 29, 2005 discloses another type of mechanism to attach the solder mass to the solder tail. According to Mongold, an electrical connector includes a connector body, a plurality of cores and a plurality of electrically conductive contacts disposed in the cores of the connector body. Each of the contacts includes a fusible member attached thereto. Each of the fusible members includes an intermediate portion and two support members disposed on opposite sides of the intermediate portion. The support members are arranged to hang down below a tail portion of the contacts. As illustrated in FIG. 1B, it looks like the solder mass 40a, 40b is attached to the contact terminal 22 in a manner of a landing gear of an airplane. According to Mongold, each fusible member 40 has two support portions 40a, 40b which are connected to each other by an intermediate portion 40c. The two support portions 40a, 40b are disposed opposite to each other and spaced from each other by a distance that is equal to a length of the intermediate portion 40c. The two support portions 40a, 40b may preferably have substantially flattened bottom surfaces as shown in FIG. 1B. However, the bottom surfaces of the support portions 40a, 40b may also have other shapes such as rounded, spherical, conical, square, rectangular, and other suitable shapes.
China Utility Model Patent No. CN2618319Y published on May 26, 2004 discloses an arrangement in which both the contact and housing is used to hold the solder mass thereto. This arrangement is similar to what illustrated by Seidler, and Schnell, while the housing of the connector body is also used.
U.S. Pat. No. 6,572,397 issued to Ju on Jun. 3, 2003 discloses another arrangement in which the solder mass is held by a cuverlinear portion of a solder tail.
US Pat. Pub. No. 20070293060 submitted by Ju discloses another arrangement in which a cradle-shaped portion is used to hold the solder mass.
China Utility Model Patent No. CN2718822Y published on Aug. 17, 2005 discloses an arrangement in which two contact terminals are arranged within a single passageway and a solder ball is held by two solder tails of the contact terminals.
On the other hand, the use of gold on electrical contacts, specially on the solder portion is also well established in the electronic industry. Gold's high reliability under repeated use, its resistance to corrosion, and low contact resistance, makes it an outstanding material for coating electrical contacts, especially those used in low voltage devices. Gold is traditionally applied to electrical contacts by electroplating the gold from aqueous solutions of gold complexes, usually cyanides or chlorides. The electronics industry in response to escalating gold prices and ever increasing economic pressures has developed sophisticated equipment for continuous and selective plating of gold in spots and stripes on strips of metal components. There are, however, a number of problems associated with electroplating gold, such as contamination of the baths accompanied by the codeposition of undesirable materials on the contacts; restriction of the range of current usable to obtain optimum plating thus limiting the speed at which components may be plated; waste due to excessive coverage; and hazards associated with the use of such poisonous compounds as potassium cyanide. Concomitant with these are the associated problems of the disposal of the hazardous industrial waste.
Mechanic Ball Attachment (MBA) is a technology different what disclosed on U.S. Pat. No. 6,024,584 issued to Lekme on Feb. 15, 2000 along with its patent family discloses how to resist solder wicking along the contact by means of nickel layer or other mechanical measurements or even chemicals. In the MBA-type socket connector, the solder ball is merely held by a mechanic force between the solder tail and housing. It is simple, and easy to make. Nevertheless, the down side is since the flux deployed onto the substrate has a limited capability to travel over the solder ball, and finally reaches to the area between the solder ball and the solder tail. Accordingly, cold joint, or poor soldering are frequently happened and encountered by the MBA socket connector. However, few attempts have been given, less to say successful result can be reached.
Ironically, it is believed that China Utility Model Patent No. CN2842814Y issued to Chen (Chen '814 patent) on Nov. 29, 2005 discloses a potential technology to resolve the above described issue by applying solder paste between solder balls and corresponding solder tails, see FIGS. 2, 3, 4, and 5. However, Chen does not detailedly and thoroughly disclose how those solder paste is applied between the solder ball and the cavity of the housing. In the above described Chen '814 patent, a lower portion of the contact terminal is almost completely located within the passageway, i.e. the passageway is completely blocked by the solder ball, rending it is difficulty or even impossible and inconvenient to apply the solder paste onto the lower portion located inside of the passageway. Practically assuming, it is impossible to apply the solder paste after the solder ball is attached. In addition, the solder paste is sticky, and if additional or excessive solder paste leaks out from the lower portion, their sticky property may create lots of mess during handling and delivery. If the solder paste extends or wicks way up into a middle portion of the contact terminal, then it would be very much likely that the geometry of the contact terminal will be altered and negate proper and intended function of the contact terminals.
It is desirable to provide an improved socket connector with a fusible element immovably connected to a soldering portion of a contact terminal and further with dried oxidation retarding and solder affinity matter deployed on the fusible element, and/or on the soldering portion, and/or between the fusible element and the soldering portion so as to achieve robust welding quality.